Image processing apparatus

ABSTRACT

An image processing apparatus includes a photographing condition estimation unit for estimating a photographing condition of the input image on the basis of photometric information of a photometric evaluation unit and focal information of a focal point detection unit; a Y/C separation unit for separating the input image into a luminance signal and a color difference signal; a luminance correction unit for extracting an edge from the luminance signal and correcting the luminance signal by a gradation conversion curve obtained by weighting depending on the photographing condition; a color difference correction unit for performing correction on the color difference signal on the basis of luminance signals obtained before and after the gradation correction and a theoretical limit characteristic of color reproduction; a skin color correction unit for performing skin color correction or the like depending on the photographing condition; a Y/C synthesis unit for synthesizing the luminance signal and the color difference signal obtained after the correction. The image processing apparatus performs gradation correction on the input image such that appropriate hue and saturation are achieved while emphasizing a main object.

This application claims benefit of Japanese Application No. 2000-044903filed in Japan on Feb. 22, 2000, and Japanese Application No.2000-044904 filed in Japan on Feb. 22, 2000, the contents of which areincorporated by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus and, morespecifically, to an image processing apparatus for adjusting a gradationrange of an input image.

2. Related Art Statement

In an image processing apparatus for synthesizing a plurality of imagespicked up under different exposure conditions to generate one widedynamic range image, a technique for adjusting a gradation range isused. For example, in Japanese Patent Application No. 11-338551 whichhas not been published yet, an image process device for dividing each ofimages into an appropriate exposure region and an inappropriate exposureregion, performing gradation correction for each appropriate exposureregion, and synthesizing the appropriate exposure regions of the imagessubjected to the gradation correction with each other to generate onewide dynamic range image is disclosed. In addition, as an example of anapparatus to which the image processing apparatus is applied, a superlatitude digital camera being capable of picking up an image of anobject in a wider dynamic range is described.

A gradation converting process in the image processing apparatusdescribed above is performed on the basis of histogram flattening of anedge portion. This histogram flattening is a technique which is on theassumption that a main object as a large number of edges, the otherportions such as a background has a small number of edges.

On the other hand, in a conventional digital camera, a color differencesignal is also converted on the basis of a coefficient obtained when aluminance signal is subjected to gradation conversion. Morespecifically, it is assumed that a luminance signal Y_(org) is convertedby a gradation conversion characteristic F such that Y_(tra)=F (Y_(org))is satisfied. In this case, conventionally, a conversion coefficientgain of a luminance signal is calculated by:

gain=Y _(tra) /Y _(org), and

the conversion coefficient is directly used to convert color differencesignals as follows:

Cb _(tra)=gain·Cb _(org)

Cr _(tra)=gain·Cr _(org).

In a digital camera, a bit count obtained when an image signal output asan analog signal from a CCD into a digital signal is often a bit countobtained when the image signal is processed and then recorded on, e.g.,a recording medium.

In this case, even though a single input image is handled to performconversion of an amount of information, especially, a reduction of anamount of information, the gradation range of the image may have to beadjusted.

The technique of gradation correction is popularly handled in variousdevices such as a printer device or a monitor device which is notlimited to the digital camera and processes an image. The abovetechnique can be applied to not only a case in which a plurality ofimages are handled but also a case in which a single input image ishandled.

The technique of histogram flattening on the assumption that the mainobject as described above has a large number of edges can correspond toan object in a relatively wide range. However, the technique cannotcompletely correspond to the object. As an example which may correspondto the exception, a case in which a person is photographed in arelatively small size on a background having a plurality of shapes orcontours is used. At this time, the background is decided as a mainobject to detect a large number of edges from the background portion,and a gradation range which is assigned to a person is reduced.

In a technique which performs gradation conversion to the colordifference signal as described above by using a conversion coefficientequal to the conversion coefficient of a luminance signal, unnaturalcolors may be generated by a high-luminance portion. More specifically,color reproduction in a color space (e.g., Y, Cb, and Cr space) has atheoretical limit characteristic (see FIG. 11 showing an embodiment ofthe present invention). The theoretical limit characteristic of thecolor reproduction is a characteristic in which a color difference rangein which colors can be reproduced increases as a luminance Y isincreased, and a color difference range in which colors can bereproduced when the luminance exceeds the luminance Y. Morespecifically, a color reproduction range is narrow because the colorsbecome blackish as a whole at a low luminance, colors in a wide rangecan be reproduced at an appropriate luminance, and a color reproductionrange becomes narrow again because the colors becomes whitish as a wholeat a high luminance.

When gradation conversion is performed like the case of luminancewithout consideration of the color reproduction range, the colorreproduction range is close to or exceeds the limit of the colorreproduction range, and colors after the gradation conversion may bewhitish. In order to cope with the problem, a process of suppressing asaturation of a high-luminance portion is added in a conventionaltechnique. However, it is not sufficient, and the realization of atechnique of more improving color reproduction is desired.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image processingapparatus being capable of adaptively adjusting a gradation range of amain object in accordance with a photographic scene.

It is another object of the present invention to provide an imageprocessing apparatus being capable of adjusting a more optimumsaturation in consideration of the theoretical limit characteristic ofcolor reproduction.

In short, the present invention is an image processing apparatus, foradjusting a gradation range of an input image, including: photographingcondition estimation means for estimating a photographing condition ofthe input image; and gradation correction means for performing gradationcorrection on the input image on the basis of the photographingcondition to adjust the gradation range to a predetermined gradationrange.

The present invention is an image processing apparatus, for adjusting agradation range of an input image, including: luminance-color differenceseparation means for separating the input image into a luminance signaland a color difference signal; gradation correction means for performinggradation correction on the luminance signal to adjust the gradationrange to a predetermined gradation range; color difference correctionmeans for correcting the color difference signal on the basis of aluminance signal obtained before the gradation correction and outputfrom the luminance-color difference separation means, a luminance signalobtained after the gradation correction, and a theoretical limitcharacteristic of color reproduction; and luminance-color differencesynthesis means for synthesizing the luminance signal obtained after thegradation correction and a color difference signal obtained after thecorrection into an image signal.

Furthermore, the present invention is also directed to an imageprocessing apparatus for adjusting a gradation range of an input image,including: photographing condition estimation means for estimating aphotographing condition of the input image; luminance-color differenceseparation means for separating the input image into a luminance signaland a color difference signal; gradation correction means for performinggradation correction on the luminance signal on the basis of thephotographing condition to adjust the gradation range to a predeterminedgradation range; color difference correction means for performingcorrection on the color difference signal on the basis of a luminancesignal obtained before the gradation correction and output from theluminance-color difference separation means, a luminance signal obtainedafter gradation correction and output from the gradation correctionmeans, and a theoretical limit characteristic of color reproduction; andluminance-color difference synthesis means for synthesizing an imagesignal on the basis of the luminance signal obtained after the gradationcorrection and a color difference signal obtained after the correction.

The present invention is also directed to an image processing apparatusfor processing an image group comprising a plurality of images obtainedby image pickup to the same object under different exposure conditionsto generate one wide dynamic range image, including: photographingcondition estimation means for estimating a photographing condition;extraction means for extracting an appropriate exposure region on thebasis of an image signal level of each image in the image group;gradation correction means for performing gradation correction on theappropriate exposure region on the basis of the photographing condition;and synthesis means for synthesizing appropriate exposure regionssubjected to gradation correction by the gradation correction means togenerate one wide dynamic range image.

In addition, the present invention is directed to an image processingapparatus for processing an image group comprising a plurality of imagesobtained by image pickup to the same object under different exposureconditions to generate one wide dynamic range image, including:luminance-color difference separation means for separating an imagesignal of each image in the image group into a luminance signal and acolor difference signal; extraction means for extracting an appropriateexposure region on the basis of a signal level of the luminance signal;gradation correction means for performing gradation correction on aluminance signal of the appropriate exposure region; color differencecorrection means for performing correction on a color difference signalof the appropriate exposure region on the basis of a luminance signalobtained before the gradation correction and output from theluminance-color difference separation means, a luminance signal obtainedafter gradation correction and output from the gradation correctionmeans, and a theoretical limit characteristic of color reproduction;luminance-color difference synthesis means for synthesizing an imagesignal on the basis of the luminance signal obtained after the gradationcorrection and a color difference signal obtained after the correction;and synthesis means for synthesizing the synthesized image signal of theappropriate exposure regions to generate one wide dynamic range image.

The present invention is still further directed to an image processingapparatus for processing an image group comprising a plurality of imagesobtained by image pickup to the same object under different exposureconditions to generate one wide dynamic range image, including:photographing condition estimation means for estimating a photographingcondition; luminance-color difference separation means for separating animage signal of each image in the image group into a luminance signaland a color difference signal; extraction means for extracting anappropriate exposure region on the basis of a signal level of theluminance signal; gradation correction means for performing gradationcorrection on the luminance signal of the appropriate exposure region onthe basis of the photographing condition; color difference correctionmeans for performing correction on a color difference signal of theappropriate exposure region on the basis of a luminance signal obtainedbefore the gradation correction and output from the luminance-colordifference separation means, a luminance signal obtained after gradationcorrection and output from the gradation correction means, and atheoretical limit characteristic of color reproduction; luminance-colordifference synthesis means for synthesizing an image signal on the basisof the luminance signal obtained after the gradation correction and acolor difference signal obtained after the correction; and synthesismeans for synthesizing the synthesized image signal of the appropriateexposure regions to generate one wide dynamic range image.

In addition, the present invention is directed to a recording medium onwhich a process program executed by a computer is recorded, wherein theprocess program adjusts a gradation range of an input image and includesthe step of separating the input image into a luminance signal and acolor difference signal; the step of performing gradation correction onthe luminance signal to adjust the gradation range to a predeterminedgradation range; the step of performing correction on the colordifference signal on the basis of a luminance signal obtained before thegradation correction, a luminance signal obtained after the gradationcorrection, and a theoretical luminance characteristic of colorreproduction; and the step of synthesizing an image signal on the basisof the luminance signal obtained after gradation correction and a colordifference signal obtained after the correction.

The present invention is also directed to a recording medium on which aprocess program executed by a computer is recorded, wherein the processprogram processes an image group comprising a plurality of imagesobtained by image pickup performed to the same object under differentexposure conditions to generate one wide dynamic range image andincludes: the step of separating an image signal of each image in theimage group into a luminance signal and a color difference signal; thestep of extracting an appropriate exposure region on the basis of asignal level of the luminance signal; the step of performing gradationcorrection on a luminance signal of the appropriate exposure region; thestep of performing correction on a color difference signal of theappropriate exposure region on the basis of a luminance signal obtainedbefore the gradation correction, a luminance signal obtained aftergradation correction, and a theoretical limit characteristic of colorreproduction; the step of synthesizing an image signal on the basis ofthe luminance signal obtained after the gradation correction and a colordifference signal obtained after the correction; and the step ofsynthesizing the synthesized image signal of the appropriate exposureregions to generate one wide dynamic range image.

These objects and advantages of the present invention will becomefurther apparent from the following detailed explanation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a basic configuration of an electroniccamera according to the first embodiment of the present invention.

FIG. 2 is a block diagram showing a detailed configuration of aphotographing condition estimation unit according to the firstembodiment.

FIG. 3 is a block diagram showing a detailed configuration of agradation correction unit according to the first embodiment.

FIG. 4 is a diagram showing an example of a divisional pattern for anevaluation photometric operation in the first embodiment.

FIG. 5 is a table showing a classification pattern of scenes from AFinformation and AE information in the first embodiment.

FIGS. 6A to 6F are diagrams showing weight coefficients obtained in edgehistogram calculation on the basis of the classification pattern shownin FIG. 5 in the first embodiment.

FIG. 7 is a flow chart showing an image conversion process in the firstembodiment.

FIG. 8 is a block diagram showing a basic configuration of an electroniccamera according to the second embodiment of the present invention.

FIG. 9 is a block diagram showing a detailed configuration of aluminance correction unit according to the second embodiment.

FIG. 10 is a block diagram showing a detailed configuration of a colordifference correction unit according to the second embodiment.

FIG. 11 is a graph showing a manner for performing color differencecorrection in consideration of a theoretical limit characteristic ofcolor reproduction in the second embodiment.

FIG. 12 is a flow chart showing an image conversion process in thesecond embodiment.

FIG. 13 is a block diagram showing a basic configuration of anelectronic camera according to the third embodiment of the presentinvention.

FIG. 14 is a flow chart showing an image conversion process in the thirdembodiment.

FIG. 15 is a block diagram showing a basic configuration of anelectronic camera according to the fourth embodiment of the presentinvention.

FIG. 16 is a block diagram showing a detailed configuration of aphotographing condition estimation unit according to the fourthembodiment.

FIG. 17 is a block diagram showing a detailed configuration of aconversion characteristic calculation unit according to the fourthembodiment.

FIG. 18 is a flow chart showing an image conversion process according tothe fourth embodiment.

FIG. 19 is a block diagram showing a basic configuration of anelectronic camera according to the fifth embodiment of the presentinvention.

FIG. 20 is a block diagram showing a detailed configuration of aluminance correction unit according to the fifth embodiment.

FIG. 21 is a block diagram showing a detailed configuration of a colordifference correction unit according to the fifth embodiment.

FIG. 22 is a block diagram showing a basic configuration of anelectronic camera according to the sixth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to the accompanying drawings.

FIGS. 1 to 7 show the first embodiment of the present invention, whereinFIG. 1 is a block diagram showing a basic configuration of an electroniccamera.

This embodiment is obtained by applying an image processing apparatusfor adjusting a gradation range according to the present invention to anelectronic camera.

This electronic camera comprises: a CCD 4 comprising a color CCD or thelike of a single CCD type having an electronic shutter function or thelike and for photoelectrically converting an object image as an imagesignal; a lens system 1 for focusing the object image on the CCD 4; adiaphragm 2 for controlling a passing range of a flux of light passingthrough the lens system 1; a low-pass filter 3 comprising an opticalfilter for removing an unnecessary high-frequency component from theflux of light passing through the diaphragm 2; an A/D converter 5 forconverting an analog image signal which is output from the CCD 4, fromwhich a noise component is removed by a correlative duplex samplingcircuit or the like (not shown), and which is subjected to amplificationinto a digital signal; an image buffer 6 for storing image data of onescreen digitized by the A/D converter 5; a photometric evaluation unit 7for reading image data from the image buffer 6 to calculate a luminancedistribution and for controlling the aperture diameter of the diaphragm2 and the electronic shutter of the CCD 4 such that an appropriateexposure is obtained in a photographing state; a focal point detectionunit 8 for reading image data from the image buffer 6 to detect a focalposition and for controlling an AF motor 9 (to be described later) onthe basis of the detection result; an AF motor 9 controlled by the focalpoint detection unit 8 to drive an AF lens of the lens system 1 and forfocusing an object image on the CCD 4; an interpolation unit 10 forinterpolating image data of a single CCD read from the image buffer 6and converting tree-CCD image data; an operation buffer 11 for storingthe image data after interpolation; a Y/C separation unit 12 serving asa luminance-color difference separation means for separating three-CCDimage data read from the operation buffer 11 into a luminance signal Yand color difference signals Cb and Cr; a photographing conditionestimation unit 13 serving as a photographing condition estimation meansfor estimating a photographing condition (as will be described below) onthe basis of photometric information output from the photometricevaluation unit 7 and focal information output from the focal pointdetection unit 8; a gradation correction unit 14 serving as a gradationcorrection means for reading a luminance signal Y from the Y/Cseparation unit 12 to extract an edge component, for performingweighting used when the histogram of an edge which is an amount ofinformation is calculated with reference to the estimation resultobtained by the photographing condition estimation unit 13 to calculatea conversion characteristic, and for performing gradation conversion ofthe luminance signal Y and the color difference signals Ob and Cr outputfrom the Y/C separation unit 12; an output unit 15 for converting animage the gradation range of which is adjusted by the gradationcorrection unit 14 into the original RGB signal or the like and thenoutputting the RGB signal to, e.g., a recording medium or a displaydevice; and a control unit 16 for receiving detection results of thephotometric evaluation unit 7 and the focal point detection unit B andcontrolling the entire electronic camera including the interpolationunit 10, the photographing condition estimation unit 13, and thegradation correction unit 14.

The focal (AF) information output from the focal point detection unit 8and input through the control unit 16 is input to a focal positionestimation unit 20 serving as a focal position estimation means andclassified into one of three types (see FIG. 5), e.g., a scenicphotographing operation (5 m to ∞), a portraiture photographingoperation (1 m to 5 m), and a close-up photographing operation (1 m orless) according to an object distance.

The photometric (AE) information output from the photometric evaluationunit 7 and input through the control unit 16 is input to an objectdistribution estimation unit 21 serving as an object distributionestimation means, and the luminance distributions of the photometricinformation are classified into several types.

More specifically, the photometric evaluation unit 7 classifies regionson the CCD 4 into 13 regions as shown in, e.g., FIG. 4, to perform adivisional photometric operation. FIG. 4 is a diagram showing adivisional pattern for an evaluation photometric operation.

That is, the central region, the left adjacent region, and the rightadjacent region of the centermost portion are represented by a₁, a₂, anda₃, respectively.

In addition, the upper and lower regions of the region a₁ in an innerperipheral portion surrounding the centermost portion are represented bya₄ and a₅, the left and right regions of the region a₄ are representedby a₆ and a₇, and the left and right regions of the region a₅ arerepresented by a₈ and a₉.

The upper left region, the upper right region, and the lower leftregion, and the lower right region in an outer peripheral portionsurrounding the inner peripheral portion are represented by a₁₀, a₁₁,a₁₂, and a₁₃.

In a divisional photometric operation using these regions, the objectdistribution estimation unit 21 calculates the following evaluationparameters:

S ₁ =|a ₂ −a ₃|  [Equation 1]

S ₂=max(|a ₄ −a ₆ |, |a ₄ −a ₇|)  [Equation 2]

S ₃=max(a ₁₀ , a ₁₁)−Av  [Equation 3]

Av=(Σa _(i))/13  [Equation 3]

More specifically, the evaluation parameter S₁ indicates the differencebetween the luminances of the left and right regions of the centermostportion, the evaluation parameter S₂ indicates a large one of thedifferences between the luminances of the upper central region and theupper left and upper right regions of the inner peripheral portion, andthe evaluation parameter S₃ indicates the difference between a large oneof the luminances of the upper left and upper right regions of the outerperipheral portion and an average luminance of an entire screen.

These evaluation parameters are obtained from the object distributionestimation unit 21, and classification of the focal positions isobtained by the focal position estimation unit 20, so that integralclassification as shown in FIG. 5 is performed in the integration unit22 serving as an integration means. FIG. 5 is a table showing aclassification pattern of scenes on the basis of AF information and AEinformation.

As shown in FIG. 5, when the AF information indicates 5 m to ∞, a scenicphotographing operation is defined, and the evaluation parameter S₃ iscompared with a predetermined value Th₁. At this time, when theevaluation parameter S₃ is larger than the predetermined value Th₁, atleast one of the regions a₁₀ and a₁₁ has a luminance which is equal toor larger than the average luminance of the entire screen to someextent. For this reason, the photographing operation is determined as aphotographing operation for a landscape containing sky on the upper side(Type 1). On the other hand, when the evaluation parameter S₃ is smallerthan the predetermined value Th₁, the photographing operation isdetermined as a photographing operation for a landscape containing nosky on the upper side or a landscape containing slight sky on the upperside (Type 2).

When the AF information indicates 1 m to 5 m, a portraiturephotographing operation is determined, and the evaluation parameter S₂is compared with a predetermined value Th₂. At this time, when theevaluation parameter S₂ is larger than the predetermined value Th₂, thephotographing operation is determined as a photographing operation for aportrait of a single person (Type 3). When the evaluation value S₂ issmaller than the predetermined value Th₂, the photographing operation isdetermined as a photographing operation for a portrait of plural persons(Type 4).

In addition, when the AF information indicates 1 m or less, thephotographing operation is determined as a close-up photographingoperation, and the evaluation parameter S₁ is compared with apredetermined value Th₃. At this time, when the evaluation parameter S₁is larger than the predetermined value Th₃, the photographing operationis determined as a close-up photographing operation for a single subject(Type 5). On the other hand, when the evaluation parameter S₁ is smallerthan the predetermined value Th₃, the photographing operation is aclose-up photographing operation for plural subjects (Type 6).

Results obtained by classifying the scenes into these types are outputfrom the integration unit 22 to the gradation correction unit 14.

Subsequently, FIG. 3 is a block diagram showing a detailed configurationof the gradation correction unit 14.

When a luminance signal Y from the Y/C separation unit 12 is input tothe edge detection unit 26 serving as a characteristic amountcalculation means, an edge detection unit 26 performs edge detection onthe basis of the control of the control unit 16. More specifically, theedge detection unit 26 comprises a general edge detection operator suchas a Laplacian or a Sobel. When the strength obtained by the edgedetection operator is equal to or larger than a predetermined thresholdvalue, binary information representing that an edge exists at areference position is output. Otherwise, the binary informationrepresents that no edge exists at the reference position.

On the other hand, when the results classified into types by thephotographing condition estimation unit 13 are input to a patternselection unit 24 serving as a selection means, the pattern selectionunit 24 based on the control unit 16 selects a weight pattern dependingon a type from a weight pattern ROM 25 in which a plurality of weightpatterns as shown in FIGS. 6A to 6F are stored in advance. FIGS. 6A to6F are diagrams showing weight coefficients obtained in histogramcalculation based on the classification pattern shown in FIG. 5. FIG. 6Ashows a weight pattern corresponding to the Type 1, FIG. 6B shows aweight pattern corresponding to the Type 2, FIG. 6C shows a weightpattern corresponding to the Type 3, FIG. 6D shows a weight patterncorresponding to the Type 4, FIG. 6E shows a weight patterncorresponding to the Type 5, and FIG. 6F shows a weight patterncorresponding to the Type 6.

In this manner, a histogram generation unit 27 serving as a histogramgeneration means calculates an edge histogram representing an appearancefrequency for luminance levels with respect to pixels constituting anedge and pixels near these pixels on the basis of the results outputfrom the edge detection unit 26. When the histogram is generated, thehistogram is calculated such that weight corresponding to pixelpositions in an image as shown in FIGS. 6A to 6F. In addition, in thehistogram generation unit 27, the calculated edge histogram isintegrated to be converted into an accumulated edge histogram.

In a conversion curve calculation unit 28 serving as a gradationconversion curve calculation means, an edge histogram is convoluted byusing a gaussian kernel or the like to generate a target histogram. Byusing the target histogram and the accumulated edge histogram outputfrom the histogram generation unit 27, a tone curve serving as agradation correction characteristic is calculated.

The conversion unit 29 serving as a conversion means performs gradationcorrection based on a tone curve obtained by the conversion curvecalculation unit 28 to image data input from the Y/C separation unit 12,and image data after the gradation correction is output to an outputunit 15. In this conversion unit 29, gradation correction of theluminance signal Y is performed, and gradation corrections of the colordifference signals Cb and Cr are sequentially performed. The obtainedsignals are output to the output unit 15.

The output unit 15 receives the luminance signal Y and the colordifference signals Cb and Cr subjected to gradation correction ongenerate, e.g., an original RGB signal and to output the original RGBsignal.

FIG. 7 is a flow chart showing an image conversion process.

An object image focused on the CCD 4 comprising a single CCD isconverted into an image signal by a photographing operation to outputthe image signal.

This image signal is converted into a digital signal by the A/Dconverter 5 and is then temporarily stored in the image buffer 6.

The photometric evaluation unit 7 and the focal point detection unit 8output AE information and AF information to the control unit 16, asdescribed above, on the basis of the image data accumulated in the imagebuffer 6 (step S1).

On the other hand, the image data stored in the image buffer 6 istransmitted to the interpolation unit 10 and interpolated with respectto R image data, G image data, and B image data. The image data isconverted into three-CCD image data (step S2), and the three-CCD imagedata is stored in the operation buffer 11.

In the Y/C separation unit 12, RGB image data is read from the operationbuffer 11 to calculate the luminance signal Y and the color differencesignals Cb and Cr as expressed by the following Equation 4 (step S3):

Y=0.29900R+0.58700G+0.14400B

Cb=−0.16874R−0.33126G+0.50000B

Cr=0.50000R−0.41869G−0.08131B  [Equation 4]

The gradation correction unit 14, in the edge detection unit 26,operates a known quadratic differential filter such as a Laplacian tothe luminance signal Y to extract an edge component (step S4). Athreshold value which is about twice a standard deviation is set for theextracted edge component to perform a binarizing process (step S5).

On the other hand, in the photographing condition estimation unit 13,the above photographing condition is estimated on the basis of the AFinformation and the AE information (step S6), and one of Type 1 to Type6 corresponding to a weight pattern is selected (step S7). A weightcoefficient corresponding to the selected weight pattern as shown inFIGS. 6A to 6F is read from the weight pattern ROM 25 (step S8).

An edge histogram is formed by the histogram generation unit 27 on thebasis of the edge component binarized in step S5 and the weight patternread in step S8 (step S9). From the edge histogram, an accumulated edgehistogram is generated (step S10).

On the basis of the edge histogram obtained as described above, theconversion curve calculation unit 28 calculates a gradation conversioncurve (step S11).

In the subsequent conversion unit 29, the luminance signal Y and thecolor difference signals Cb and Cr output from the Y/C separation unit12 are subjected to a conversion process by the gradation conversioncurve obtained by the conversion curve calculation unit 28 (step S12),so that the converted image data is output (step S13).

Although both the photometric information and the focal information areused to estimate the photographing conditions in the above description,the photographing condition may be estimated by using only one of thephotometric information and the focal information to change weighting.If not only the photometric information and the focal information areused, but also one or more of zoom position information, multi-spotphotometric information, line-of-sight input information, emissioninformation of an strobe flash, information of a detection sensor fordetecting the vertical and horizontal positions of an electronic camera,and white balance information are referred to, a photographing conditioncan be more exactly estimated.

In addition, the above described technique of gradation correctiondepending on the photographing condition is applied to not only a colorimage but also a black-and-white image.

In this first embodiment, gradation correction depending on aphotographing condition is performed by the image processing apparatuswhich is included as a circuit in an electronic camera. However, theseprocesses can also be performed by a process program of a computer. Inthis case, photographing information such as photometric information andfocal information is recorded on, e.g., a header portion of an imagefile in a computer, a photographing condition is estimated on thesepieces of photographing information, so that gradation correctionappropriate to the photographing condition may be performed.

The image processing apparatus is not only applied to an electroniccamera, but also popularly applied to devices such as printer deviceswhich handle images.

According to the first embodiment, a photographing condition is decidedon the basis of photographing information such as focal information andphotometric information, and weighting depending on the photographingcondition is performed when an edge histogram is formed. For thisreason, gradation correction which is maximally appropriate to aphotographing scene can be performed in consideration of a main object.

FIGS. 8 to 12 show the second embodiment of the present invention,wherein FIG. 8 is a block diagram showing a basic configuration of anelectronic camera, FIG. 9 is a block diagram showing a detailedconfiguration of a luminance correction unit, FIG. 10 is a block diagramshowing a detailed configuration of a color difference correction unit,FIG. 11 is a graph showing a manner for performing color differencecorrection in consideration of a theoretical limit characteristic ofcolor reproduction, and FIG. 12 is a flow chart showing an imageconversion process.

The same reference numerals as in the first embodiment described abovedenote the same parts in the second embodiment, and a descriptionthereof will be omitted. Only different points will be mainly describedbelow.

In the second embodiment, the luminance signal Y and the colordifference signals Cb and Cr separated by the Y/C separation unit 12 areinput to a luminance correction unit 17 serving as a gradationcorrection means and a color difference correction unit 18 serving as acolor difference correction means, respectively.

The luminance correction unit 17, as shown in FIG. 9, receives theluminance signal Y output from the Y/C separation unit 12, and performsgradation correction for luminance. The luminance correction unit 17comprises an edge extraction unit 30 serving as characteristic amountcalculation means, a histogram generation unit 31 serving as a histogramgeneration means, a conversion curve calculation unit 32 serving as agradation conversion curve calculation means, and a luminance conversionunit 33 serving as a luminance conversion means.

The processes in the luminance correction unit 17 will be described withreference to FIG. 12.

The luminance correction unit 17 reads the luminance signal Y outputfrom the Y/C separation unit 12 (step S21), the edge extraction unit 30operates a filter such as a Laplacian to extract an edge component (stepS22). The edge component is compared with a predetermined thresholdvalue with respect to pixels, and a binarizing process is conducted fordetermining whether a pixel is an edge or not (step S23).

On the basis of information output from the edge extraction unit 30, thehistogram generation unit 31 generates an edge histogram representing anappearance frequency of an edge to a luminance (step S24), and the edgehistogram is integrated to generate an accumulated edge histogram (stepS25).

The conversion curve calculation unit 32 calculates a tone curve servingas a gradation correction characteristic as described above by using theaccumulated edge histogram output from the histogram generation unit 31(step S26)

The luminance conversion unit 33 performs gradation conversion to theluminance signal Y on the basis of the conversion curve under thecontrol of the control unit 16 (step S27), outputs the convertedluminance signal Y to the color difference correction unit 18, andoutputs the converted luminance signal Y to a Y/C synthesis unit 19serving as a luminance-color difference synthesis means (step S28).

In this manner, it is assumed that a luminance signal obtained beforethe gradation correction and output from the Y/C separation unit 12 isrepresented by Y_(org) and that a luminance signal subjected to afterthe gradation correction by the luminance conversion unit 33 isrepresented by Y_(tra). In this case, the luminance signals Y_(org) andY_(tra), as described below, are used when gradation of a colordifference is corrected in the color difference correction unit 18.

The color difference correction unit 18, as shown in FIG. 10, receivesthe color difference signals Cb and Cr output from the Y/C separationunit 12 to perform gradation correction on a color difference. The colordifference correction unit 18 comprises a first correction coefficientcalculation unit 37 serving as a first calculation means, a secondcorrection coefficient calculation unit 35 serving as a secondcalculation means, a color reproduction limit characteristic ROM 36, anda color difference conversion unit 38 serving as a color differenceconversion means.

In the color difference correction unit 18, the first correctioncoefficient calculation unit 37 receives a luminance signal Y_(org)obtained before gradation correction from the Y/C separation unit 12,and calculates a color reproduction range b_(org) corresponding to theluminance signal Y_(org) as expressed in the following Equation 5 (stepS31):

b _(org) =B (Y _(org))  [Equation 5]

In this equation, a function B(Y) is a function representing atheoretical limit characteristic of color reproduction in a color space(Y, Cb, and Cr space), and has the following characteristic. That is,for example, as typically shown in FIG. 11, a color difference range inwhich color reproduction can be performed is widened as the luminance Yis increased, and the color difference range in which color reproductioncan be performed is narrowed when the luminance Y exceeds a certainluminance.

The calculation as described in Equation 5 is performed such that acolor reproduction range b_(org) corresponding to the luminance signalY_(org) is calculated with reference to table data or the like stored inthe color reproduction limit characteristic ROM 36 (step S30).

The color reproduction limit characteristic ROM 36 stores the functionB(Y) representing the theoretical limit characteristic of colorreproduction as table data in advance. In this case, the table datawhich is changed into a ROM in consideration of a load, a processingrate, and the like obtained by calculation is used. However, the tabledata may be obtained by actual calculation.

The second correction coefficient calculation unit 35 receives theluminance signal Y_(tra) obtained after gradation correction and outputfrom the luminance correction unit 17 to calculate a color reproductionrange b_(tra) corresponding to the luminance signal Y_(tra) as describedin the following Equation 6 which is similar to the Equation 5 describedabove (step S32):

b _(tra) =B(Y _(tra))  [Equation 6]

The calculation described in Equation 6, similarly, is performed suchthat the color reproduction range b_(tra) corresponding to the luminanceY_(tra) is calculated with reference to table data or the like stored inthe color reproduction limit characteristic ROM 36 (step S30).

The color difference conversion unit 38 calculates a conversioncoefficient gain_(c) corresponding to a color difference signal on thebasis of the b_(org) serving as a first correction coefficient and theb_(tra) serving as a second correction coefficient as described in thefollowing Equation 7:

gain _(c) =b _(tra) /b _(org)  [Equation 7]

In this manner, the conversion coefficient gain_(c) corresponding to thecolor difference signal is defined as a ratio of the theoretical limitcharacteristic b_(org) of color reproduction in the luminance signalY_(org) obtained before gradation correction on the theoretical limitcharacteristic b_(tra) of color reproduction in the luminance signalY_(tra) obtained after gradation correction. For this reason, faithfulcolor reproduction which keeps a hue can be performed without making animage monotonous as in a case using the same conversion coefficient asthat used in calculation for a luminance.

When the conversion coefficient gain_(c) is calculated, color differencesignals Cb_(org) and Cr_(org) obtained before correction aresequentially received from the Y/C separation unit 12 (step S29), andthe color difference signals Cb_(tra) and Cr_(tra) obtained aftercorrection are calculated as described in the following Equation 8 (stepS33).

Cb _(tra)=gain_(c) ·Cb _(org)

Cr _(tra)=gain_(c) ·Cr _(org)  [Equation 8]

The color difference signals Cb_(tra) and Cr_(tra) converted in thismanner are output to the Y/C synthesis unit 19 (step S34).

In the Y/C synthesis unit 19, the luminance signal Y_(tra) obtainedafter gradation conversion and the color difference signals Cb_(tra) andCr_(tra) obtained after conversion are Y/C-synthesized to be convertedinto, e.g., an RGB signal. The RGB signal is output through the outputunit 15 under the control of the control unit 16.

In the second embodiment, gradation correction made in consideration ofa theoretical limit characteristic of color reproduction is performed byan image processing apparatus which included as a circuit in anelectronic camera. However, such these processes can be performed by aprocess program of a computer.

This image processing apparatus is not only applied to an electroniccamera, but also popularly applied to devices such as printer deviceswhich handle images.

According to the second embodiment, gradation correction made inconsideration of a theoretical limit characteristic of colorreproduction is performed to a color difference signal. For this reason,even though gradation correction of an image is performed, a hue can bekept.

FIGS. 13 and 14 show the third embodiment of the present invention,wherein FIG. 13 is a block diagram showing a basic configuration of anelectronic camera, and FIG. 14 is a flow chart showing an imageconversion process.

The same reference numerals as in the first and second embodimentsdenote the same parts in the third embodiment, and a description thereofwill be omitted. Only different points will be mainly described below.

The third embodiment is constituted such that a function obtained bycombining the functions of the first embodiment and the secondembodiment appears. More specifically, weighting depending on aphotographing condition is performed when a gradation conversioncharacteristic of a luminance signal is calculated, a theoretical limitcharacteristic of color reproduction is considered when a conversioncharacteristic of a color difference signal is calculated, and a hue anda saturation are corrected depending on the photographing condition.

More specifically, an image signal output from the CCD 4 is convertedinto a digital signal by the A/D converter 5, and then the digitalsignal is temporarily stored in the image buffer 6.

As described above, the photometric evaluation unit 7 and the focalpoint detection unit 8 outputs AE information and AF information to thecontrol unit 16 on the basis of image data accumulated in the imagebuffer 6 (step S41).

On the other hand, the image data stored in the image buffer 6 aresequentially transmitted to the interpolation unit 10 and interpolatedwith respect to R image data, G image data, and B image data to beconverted into three-CCD image data (step S42). The three-CCD image dataare stored in the operation buffer 11.

In the Y/C separation unit 12, RGB image data is read from the operationbuffer 11, and the luminance signal Y and the color difference signalsCb and Cr are calculated as described in Equation 4 described above(step S43).

The luminance correction unit 17, in the edge extraction unit 30,extracts an edge component from the luminance signal Y of these signals(step S44), and a binarizing process is performed to the extracted edgecomponent (step S45).

On the other hand, in the photographing condition estimation unit 13, aphotographing condition is estimated on the basis of the AF informationand the AE information as described above (step S46), and one of Type 1to Type 6 corresponding to a weight pattern is selected (step S47). Aweight coefficient, as shown in FIGS. 6A to 6F, corresponding to theselected weight pattern is read from the weight pattern ROM 25 (stepS48).

In this manner, an edge histogram weighted by the histogram generationunit 31 is generated on the basis of the edge component binarized instep S45 and a weight pattern read in step S8 (step S49), and anaccumulated edge histogram is further generated from the edge histogram(step S50).

The conversion curve calculation unit 32 calculates a gradationconversion curve on the basis of the accumulated edge histogram obtainedin this manner (step S51).

In the subsequent luminance conversion unit 33, the luminance signal Youtput from the Y/C separation unit 12 is subjected to a conversionprocess by the gradation conversion curve obtained from the conversioncurve calculation unit 32 (step S52). The converted luminance signal Yis output to the color difference correction unit 18 and output to theY/C synthesis unit 19.

In the color difference correction unit 18, a first correctioncoefficient and a second correction coefficient are calculated asdescribed above on the basis of a luminance signal Y′ obtained after thegradation correction, the luminance signal Y obtained before thegradation correction and output from the Y/C separation unit 12, and atheoretical limit characteristic of color reproduction read from thecolor reproduction limit characteristic ROM 36. A conversion coefficientto a color difference signal is calculated on the basis of the firstcorrection coefficient and the second correction coefficient. The colordifference signals Cb and Cr received from the Y/C separation unit 12are converted (step S53).

Color difference signals Cb′ and Cr′ corrected by the color differencecorrection unit 18 are input to a skin color correction unit 41 servingas a second color difference correction means to perform correction orthe like of, e.g., a skin color with reference to a photographingcondition estimated by the photographing condition estimation unit 13(step S54).

In this case, when a portrait of a single person or plural persons as inType 3 or Type 4 is used, a process of correcting a hue and a saturationis performed such that a skin color looks more preferable. For example,the skin color of a Japanese person is exemplified. That is, sinceJapanese skin tends to have a yellowish hue, a process of slightlyshifting the yellowish hue to a reddish hue is performed.

When a landscape containing sky on the upper side as in Type 1, aprocess of correcting a hue and a saturation is performed such that thesky looks more blue.

In addition, when a landscape is estimated as in Type 1 or Type 2, andwhen it is estimated on the basis of other information that thelandscape contains a large number of plants, a process of correcting ahue and a saturation may be performed to make the green of the plantspreferable. The embodiment can be applied to not only the above objects,but also various other objects.

The color difference signals Cb′ and Cr′ corrected by the skin colorcorrection unit 41 and the luminance signal Y′ subjected to gradationcorrection by the luminance correction unit 17 are synthesized by theY/C synthesis unit 19 to be converted into an original RGB signal (stepS55). Thereafter, the RGB signal is output through the output unit 15(step S56).

Also, in the third embodiment, the device operates a circuit in anelectronic camera to perform gradation correction. However, theseprocesses can also be performed by a process program of a computer.

In this case, photometric information from the photometric evaluationunit 7 and focal information from the focal point detection unit 8 areadded to the header portion of a file in which image data in the imagebuffer 6 is stored, and the file is recorded on a recording medium suchas a memory card. The file may be read by the computer to cause thecomputer to sequentially perform necessary parts of the processes shownin FIG. 14.

The process program is recorded on various recording media such as ahard disk in the computer, a portable floppy disk, and a photomagneticdisk.

The image processing apparatus is not only applied to an electroniccamera, but also popularly applied to devices such as printer deviceswhich handle images.

According to the third embodiment described above, both of the effect ofthe first embodiment described above and the effect of the secondembodiment described above can be achieved. At the same time, correctionof a hue and a saturation can be performed depending on a photographingcondition. For this reason, the skin color of a person which is, e.g., amain object, can also be corrected into a preferable skin color.

FIGS. 15 to 18 show the fourth embodiment of the present invention,wherein FIG. 15 is a block diagram showing a basic configuration of anelectronic camera.

The same reference numerals as in the first to third embodimentsdescribed above denote the same parts in the fourth embodiment, and adescription thereof will be omitted. Only different points will bemainly described below.

This embodiment is obtained by applying the image processing apparatusaccording to the present invention to an electronic camera. For the sakeof descriptive convenience, a case in which two images, i.e., an imageobtained by short-period exposure and an image obtained by long-periodexposure are synthesized with each other to obtain one wide dynamicrange image will be described below. As a matter of course, theembodiment can also be applied to a case in which a large number ofimages are synthesized with each other.

This electronic camera, as shown in FIG. 15, has a configuration whichis almost the same as that shown in FIG. 1. However, since theelectronic camera synthesizes a plurality of images, the electroniccamera is different from the electronic camera shown in FIG. 1 in thefollowing points.

More specifically, as image buffers for accumulating image data of onescreen digitized by the A/D converter 5, a first image buffer 6 a forstoring the image obtained by long-period exposure and a second imagebuffer 6 b for storing the image obtained by short-period exposure arearranged.

The photometric evaluation unit 7 reads image data from the first imagebuffer 6 a also used to accumulate data for a photometric operation andfocal point detection to calculate a luminance distribution. Theaperture diameter of the diaphragm 2 and the electronic shutter of theCCD 4 are controlled such that an appropriate exposure is obtained in aphotographing state.

In addition, the focal point detection unit 8 reads image data from thefirst image buffer 6 a to detect a focal position. On the basis of thedetection result, the AF motor 9 is controlled.

The interpolation unit 10 interpolates single-CCD image data read fromthe first and second image buffers 6 a and 6 b to convert the image datainto three-CCD image data.

The electronic camera further comprises: an appropriate exposureextraction unit 51 serving as an extraction means for reading theluminance signal Y from the Y/C separation unit 12 to decide, by asignal level, whether each of the pixels constituting the entire screenhas an appropriate exposure or not, and for, on the basis of the result,extracting and outputting divisional image information; a conversioncharacteristic calculation unit 52 serving as a gradation correctionmeans for performing weighting when a histogram of an edge which is acharacteristic amount with reference the estimation result obtained bythe photographing condition estimation unit 13 to calculate a conversioncharacteristic and for performing gradation conversion on theappropriate exposure region output from the appropriate exposureextraction unit 51; and an image synthesis unit 53 serving as asynthesis means for synthesizing an image related to long-periodexposure and an image related to short-period exposure which areobtained after gradation conversion and output from the conversioncharacteristic calculation unit 52 with reference to region informationoutput from the appropriate exposure extraction unit 51 to generate onewide dynamic range image. A wide dynamic range image synthesized by theimage synthesis unit 53 is output by the output unit 15 to, e.g., arecording medium or a display device.

In addition, the control unit 16 performs control of the entireelectronic camera, and also controls the appropriate exposure extractionunit 51, the conversion characteristic calculation unit 52, and theimage synthesis unit 53, as a matter of course.

FIG. 16 is a block diagram showing a detailed configuration of thephotographing condition estimation unit 13. The internal configurationin FIG. 16 is the same as that shown in FIG. 2. The internalconfiguration in FIG. 16 is different from the internal configuration inFIG. 2 only that results classified into the types described above areoutput from the integration unit 22 to the conversion characteristiccalculation unit 52.

Subsequently, FIG. 17 is a block diagram showing a detailedconfiguration of the conversion characteristic calculation unit.

The internal configuration of the conversion characteristic calculationunit 52 is almost the same as that of the gradation correction unit 14(see FIG. 3) in the first embodiment described above.

More specifically, as described above, the appropriate exposureextraction unit 51 reads the luminance signal Y of a long-periodexposure image and compares the signal level of each of pixelsconstituting the entire screen with a predetermined value to decidewhether the corresponding pixel has an appropriate exposure or not. Aset of pixels which are determined as pixels having appropriateexposures corresponds to an appropriate exposure region related tolong-period exposure, and the other portion except for the appropriateexposure region related to the long-period exposure corresponds to anappropriate exposure region related to short-period exposure.

When the luminance signal Y in the appropriate exposure region of thelong-period exposure output from the appropriate exposure extractionunit 51 is input to the edge detection unit 26 serving as acharacteristic amount calculation means, the edge detection unit 26performs edge detection under the control of the control unit 16.

The other components, i.e., the pattern selection unit 24, the weightpattern ROM 25, the histogram generation unit 27, and the conversioncurve calculation unit 28 have a configuration which is almost the sameas that of the first embodiment described above to be operated.

In addition, the conversion unit 29 serving as a conversion means is thesame as that in the first embodiment except that image data is inputfrom the appropriate exposure extraction unit 51, and the image dataobtained after gradation correction is output to the image synthesisunit 53. In the conversion unit 29, gradation correction of theluminance signal Y related to long-period exposure is performed, andgradation corrections of the color difference signals Cb and Cr relatedto the long-period exposure are sequentially performed to output thecorrected signals to the image synthesis unit 53. Thereafter, gradationcorrections of a luminance signal Y and color difference signals Cb andCr related to short-period exposure are performed in the same manner asdescribed above to output the corrected signals to the image synthesisunit 53.

The subsequent image synthesis unit 53 receives the luminance signal Yand the color difference signals Cb and Cr obtained after the gradationcorrection related to the long-period exposure to generate, e.g., an RGBsignal related to the long-period exposure. The image synthesis unit 53receives the luminance signal Y and the color difference signals Cb andCr obtained after the gradation correction related to the short-periodexposure to generate the RGB signal related to the short-periodexposure. Thereafter, these signals are synthesized with each other togenerate a wide dynamic range image, and the image is output.

FIG. 18 is a flow chart showing an image conversion process.

An object image focused on the CCD 4 which comprises a single CCD isdesigned to perform image pickup operations plural times under differentexposure conditions. As described above, an image pickup operation bylong-period exposure and an image pickup operation by short-periodexposure are performed in this order to sequentially output signals asimage signals.

These image signals are converted into digital signals by the A/Dconverter 5. Thereafter, the digital signals are stored in the firstimage buffer 6 a and the second image buffer 6 b, respectively.

The photometric evaluation unit 7 and the focal point detection unit 8outputs AE information and AF information to the control unit 16 asdescribed above on the basis of image data of long-period exposureaccumulated in the first image buffer 6 a which is one of these buffers(step S61).

On the other hand, the image data stored in the first image buffer 6 aand the second image buffer 6 b are sequentially transmitted to theinterpolation unit 10 and interpolated with respect to R image data, Gimage data, and B image data to be converted into three-CCD image data(step S62). The three-CCD image data are stored in the operation buffer11.

In the Y/C separation unit 12, RGB image data is read from the operationbuffer 11, and the luminance signal Y and the color difference signalsCb and Cr are calculated as described in Equation 4 described above(step S63).

The appropriate exposure extraction unit 51 compares the signal level ofthe luminance signal Y of these signals with a predetermined thresholdvalue with respect to pixels to decide whether the corresponding pixelbelongs an appropriate exposure region or not, so that divisional imageinformation is extracted and output (step S64).

Thereafter, the edge detection unit 26 in the conversion characteristiccalculation unit 52 operates a known quadratic differential filter suchas a Laplacian to the luminance signal Y to extract an edge component(step S65). A threshold value which is about twice a standard deviationis set for the extracted edge component to perform a binarizing process(step S66).

On the other hand, in the photographing condition estimation unit 13,the above photographing condition is estimated on the basis of the AFinformation and the AE information (step S67), and one of Type 1 to Type6 corresponding to a weight pattern is selected as shown in FIG. 5 (stepS68). A weight coefficient corresponding to the selected weight patternas shown in FIGS. 6A to 6F is read from the weight pattern ROM 25 (stepS69).

In this manner, the histogram generation unit 27 generates an edgehistogram weighted by the histogram generation unit 27 on the basis ofthe edge component binarized in step S66 and the weight pattern read instep S69 (step S70), and an accumulated edge histogram is generated fromthe edge histogram (step S71).

On the basis of the accumulated edge histogram obtained as describedabove, the conversion curve calculation unit 28 calculates a gradationconversion curve (step S72).

In the subsequent conversion unit 29, the luminance signal Y and thecolor difference signals Cb and Cr output from the appropriate exposureextraction unit 51 are subjected to a conversion process by thegradation conversion curve obtained from the conversion curvecalculation unit 28 (step S73), and the converted image data is output(step S74).

In the fourth embodiment, gradation correction corresponding to aphotographing condition is performed by an image processing apparatuswhich is included as a circuit in an electronic camera. However, theseprocesses can also be performed by a process program of a computer. Inthis case, photographing information such as photometric information andfocal information is recorded on, e.g., the header portion of an imagefile. In the computer, a photographing condition may be estimated on thebasis of these pieces of photographing information to perform gradationcorrection appropriate to the photographing condition.

The image processing apparatus is not only applied to an electroniccamera, but also popularly applied to devices such as printer deviceswhich handle images.

According to the fourth embodiment described above, as in theembodiments described above, gradation correction which is mostappropriate to a photographing scene can be performed in considerationof a main object.

FIGS. 19 to 21 show the fifth embodiment of the present invention,wherein FIG. 19 is a block diagram showing a basic configuration of anelectronic camera, FIG. 20 is a block diagram showing a detailedconfiguration of a luminance correction unit, and FIG. 21 is a blockdiagram showing a detailed configuration of a color differencecorrection unit.

The same reference numerals as in the first to fourth embodimentsdescribed above denote the same parts in the fifth embodiment, and adescription thereof will be omitted. Only different points will bemainly described below.

In the fifth embodiment, a luminance signal Y and color differencesignals Cb and Cr separated by the Y/C separation unit 12, as shown inFIG. 19, are input to the luminance correction unit 17 serving as agradation correction means and the color difference correction unit 18serving as a color difference correction means through the appropriateexposure extraction unit 51, respectively.

The luminance correction unit 17, as shown in FIG. 20, has the sameconfiguration as that shown in FIG. 9 except that the luminance signal Yof an appropriate exposure region is received from the appropriateexposure extraction unit 51. The processes of the luminance correctionunit 17 are the same as those shown in FIG. 12.

In addition, the color difference correction unit 18, as shown in FIG.21, has the same configuration as that shown in FIG. 10 except that thefirst correction coefficient calculation unit 37 and the colordifference conversion unit 38 receive outputs from the appropriateexposure extraction unit 51.

In the Y/C synthesis unit 19, a luminance signal Y_(tra) obtained aftergradation conversion and color difference signals Cb_(tra), Cr_(tra)obtained after conversion are Y/C-synthesized with each other to beconverted into, e.g., an RGB signal, and the RGB signal is output to theimage synthesis unit 53.

In the image synthesis unit 53, an appropriate exposure image portionobtained after gradation conversion related to long-period exposure andan appropriate exposure image portion obtained after gradationconversion related to short-period exposure are synthesized with eachother to generate a wide dynamic range image. Thereafter, the widedynamic range image is output from the output unit 15.

In this fifth embodiment, gradation correction made in consideration ofa theoretical limit characteristic of color reproduction is performed byan image processing apparatus which is included as a circuit in anelectronic camera. However, these processes can also be performed by aprocess program of a computer.

The image processing apparatus is not only applied to an electroniccamera, but also popularly applied to devices such as printer deviceswhich handle images.

According to the fifth embodiment, gradation correction made inconsideration of a theoretical limit characteristic of colorreproduction is performed to a color difference signal. For this reason,even though gradation correction of an image is performed, anappropriate hue can be kept.

FIG. 22 shows the sixth embodiment of the present invention, and is ablock diagram showing a basic configuration of an electronic camera.

The same reference numerals as in the first to fifth embodiments denotethe same parts in the sixth embodiment, and a description thereof willbe omitted. Only different points will be mainly described below.

The sixth embodiment is constituted such that a function obtained bycombining the functions of the fourth embodiment and the fifthembodiment appears. More specifically, weighting depending on aphotographing condition is performed when a gradation conversioncharacteristic of a luminance signal is calculated, a theoretical limitcharacteristic of color reproduction is considered when a conversioncharacteristic of a color difference signal is calculated.

More specifically, as shown in FIG. 22, the classification results ofphotographing conditions as shown in FIG. 5 estimated by thephotographing condition estimation unit 13 are input to the luminancecorrection unit 17. When an edge histogram of a luminance signal isgenerated in the luminance correction unit 17, weighting as shown inFIGS. 6A to 6F is performed.

When a luminance signal subjected to gradation conversion on the basisof the characteristic curve obtained as described above is input to thecolor difference correction unit 18, as in the fifth embodimentdescribed above, a second correction coefficient is calculated withreference to a table showing the theoretical limit characteristic ofcolor reproduction. Similarly, a first correction coefficient iscalculated on the basis of a luminance signal obtained before gradationconversion. On the basis of the first correction coefficient and thesecond correction coefficient, a conversion coefficient related to acolor difference signal is calculated. Conversion appropriate to a colordifference is performed to output the converted coefficient to the Y/Csynthesis unit 19.

In the Y/C synthesis unit 19, a luminance signal obtained aftergradation conversion and output from the luminance correction unit 17and a color difference signal obtained after conversion and output fromthe color difference correction unit 18 are Y/C-synthesized with eachother to output the synthesized signal to the image synthesis unit 53.In the image synthesis unit 53, an appropriate exposure image portionobtained after gradation conversion related to long-period exposure andan appropriate exposure image portion obtained after gradationconversion related to short-period exposure are synthesized with eachother to generate a wide dynamic range image. The wide dynamic rangeimage is output from the output unit 15.

Also in the sixth embodiment, gradation correction is performed by animage processing apparatus which is included as a circuit in anelectronic camera. However, these processes can also be performed by aprocess program of a computer.

The image processing apparatus is not only applied to an electroniccamera, but also popularly applied to devices such as printer deviceswhich handle images.

According to the sixth embodiment described above, both of the effect ofthe fourth embodiment described above and the effect of the fifthembodiment described above can be achieved.

In this invention, it is apparent that working modes different in a widerange can be formed on the basis of this invention without departingfrom the spirit and scope of the invention. This invention is notrestricted by any specific embodiment except being limited by theappended claims.

What is claimed is:
 1. An image processing apparatus for adjusting agradation range of an input image, comprising: photographing conditionestimation means for estimating a photographing condition of the inputimage; luminance-color difference separation means for separating theinput image into a luminance signal and a color difference signal;gradation correction means for performing gradation correction of theluminance signal on the basis of the photographing condition to adjustthe gradation range to a predetermined gradation range; color differencecorrection means for performing correction of the color differencesignal on the basis of a luminance signal obtained before the gradationcorrection and output from the luminance-color difference separationmeans, a luminance signal obtained after gradation correction and outputfrom the gradation correction means, and a theoretical limitcharacteristic of color reproduction; and luminance-color differencesynthesis means for synthesizing an image signal on the basis of theluminance signal obtained after the gradation correction and a colordifference signal obtained after the correction.
 2. An image processingapparatus according to claim 1, wherein the photographing conditionestimation means estimates a photographing condition on the basis of atleast one piece of information of focal information, photometricinformation, zoom position information, multi-spot photometricinformation, line-of-sight input information, and strobe flashinformation.
 3. An image processing apparatus according to claim 1,wherein the photographing condition estimation means comprises: focalposition estimation means for estimating at least three types of focalpositions including a scenic photographing operation, a portraiturephotographing operation, and a close-up photographing operation from thefocal information; object distribution estimation means for estimatingat least three types of object distributions of an entire screen, acenter focus, and a central portion from the photometric information;and integration means for integrally estimating a photographingcondition by combining the focal position estimated by the focalposition estimation means and the object distribution estimated by theobject distribution estimation means.
 4. An image processing apparatusaccording to claim 1, wherein the gradation correction means comprises:selection means for selecting an arrangement of a weight coefficient onthe basis of the photographing condition; characteristic amountcalculation means for calculating a characteristic amount with respectto the input image; histogram generation means for generating aweighting histogram related to the characteristic amount on the basis ofthe arrangement of the weight coefficient; gradation conversion curvecalculation means for calculating a gradation conversion curve on thebasis of the histogram; and conversion means for performing gradationconversion by using the gradation conversion curve.
 5. An imageprocessing apparatus according to claim 1, wherein the color differencecorrection means comprises: first calculation means for calculating afirst correction coefficient on the basis of a luminance signal obtainedbefore the gradation correction and the theoretical limit characteristicof color reproduction; second calculation means for calculating a secondcorrection coefficient on the basis of the luminance signal obtainedafter the gradation correction and the theoretical limit characteristicof color reproduction; and color difference conversion means forconverting a color difference signal by using the first correctioncoefficient and the second correction coefficient.
 6. An imageprocessing apparatus according to claim 1 further comprising: secondcolor difference correction means for correcting at least one of a hueand a saturation of a specific color in the color difference signalobtained after the correction on the basis of the photographingcondition.
 7. An image processing apparatus according to claim 1,wherein the gradation correction means adjusts a gradation range byreducing the gradation range.